The present invention relates to methods and apparatus which process data received from an ultrasound transducer in preparation for subsequent signal processing, such as, modulation recovery and scan conversion. More particularly, the present invention provides a single path for processing of data received from an ultrasound transducer, entirely in the digital domain, with an improved dynamic range.
FIG. 1 is a block diagram of an ultrasound system 100. A transmit waveform generator 110 is coupled through a transmit/receive (T/R) switch 112 to a transducer array 114, which includes an array of transducer elements. The T/R switch 112 typically has one switch element for each transducer element. The transmit waveform generator 110 receives transmit pulse timing sequences from a waveform timing generator 116. The transducer array 114, energized by the transmit waveform generator 110, transmits ultrasound energy into a region of interest in a patient""s body and receives reflected ultrasound energy, or echoes, from various structures and organs within the patient""s body. As is known in the art, by appropriately delaying the pulses applied to each transducer element by the transmit waveform generator 110, a steered and focused ultrasound beam is transmitted.
The transducer array 114 is coupled through the T/R switch 112 to a receive waveform generator 118. Ultrasound echoes from a given point within the patient""s body are received by the transducer elements at different times. The transducer elements convert the received ultrasound echoes to transducer signals which may be amplified, individually delayed and then summed by the receive waveform generator 118 to provide a waveform generator signal that represents the received ultrasound level along a desired receive line. The echoes exhibit a wide dynamic range of approximately 160 dB. Echoes from objects close to the transducer can produce a signal of 200 millivolts from the transducer, while echoes farther away from the transducer can produce a signal having an amplitude eight (8) orders of magnitude less. Any echo that cannot produce at least a 2 nano-volt response in the transducer array 114 can be lost in the noise.
The waveform generator signals are applied to a signal processor 124 which digitally processes the waveform generator signal for improved image quality and to perform such processing as color flow processing. As known in the art, the delays applied to the transducer signals may be varied during reception of ultrasound energy to effect dynamic focusing. The receive waveform generator 118 and the signal processor 124 form an ultrasound receiver 126. The output of the signal processor 124 is supplied to a scan converter 128 which converts sector scan or other scan pattern signals to conventional raster scan display signals. The output of the scan converter 128 is supplied to a display unit 130, which displays an image of the region of interest in the patient""s body. In the case of a three-dimensional scan pattern, the scan converter 128 may be replaced by an image data buffer that stores the three-dimensional data set and a processor that converts the three-dimensional data set to a desired two-dimensional image.
A system controller 132 provides overall control of the system, including timing control. The system controller 132 typically includes a microprocessor operating under the control of control routines 134 stored in a memory 136. The system controller also utilizes the memory 136 to store intermediate values, including system variables describing the operation of the ultrasound imaging system 100. An external storage 138, for example a floppy disk drive, a CD-ROM drive, a videotape unit, etc . . . , may be utilized for more permanent and/or transportable storage of data.
FIG. 2 is a block diagram of a single channel in a known ultrasound receiver. The ultrasound receiver shown herein is limited to a single receive channel so as to simplify explanation of the circuit. The elements within the dotted lines are repeated for each channel of the waveform generator, while the elements outside the dotted lines are global, serving the entire circuit.
Block 200 contains a representation of a signal produced by an element of a transducer array 114 (FIG. 1). Basically, the signal from each element can be represented as a source voltage 210 (Es) having a resistance 212 (Rs) and a noise component 214 (en). The signal is fed into a beamforming channel 205. As noted above, each element of the transducer array 114 typically (but not necessarily) has a corresponding beamforming channel 205.
The signal from each element is first amplified by amplifier 216 to bring the signal level up to an appropriate value. Subsequently, the signal is processed on two paths, one for digital processing (the xe2x80x9cdigital path,xe2x80x9d elements 218-226) and one for analog processing (the xe2x80x9canalog path,xe2x80x9d elements 232 and 234).
Looking at the digital path, the signal 200 is first applied to a filter 218, such as a harmonic filter or clipping filter. Filter 218 acts on the entire dynamic range of the signal 200 and prepares the signal for subsequent processing. In the case of the harmonic filter, echoes exhibiting a harmonic frequency of the fundamental transmitted frequency are extracted (or allowed to pass). The resultant signal is processed using so called xe2x80x9charmonic processing.xe2x80x9d
The output of the filter 218 is applied to a variable gain amplifier 220, which, in effect provides a window into the dynamic range of the signal 200. In other words, the variable gain amplifier selects a portion of the dynamic range for subsequent processing. The portion of the signal 200 selected is varied based on the type of A/D converter used (121 dB wide for an 8 bit A/D converter and 133 dB wide for a 10 bit A/D converter at 40 MSPS sample rates) and the subsequent image processing to be applied to the signal 200.
The digital processing path utilizes a pulse mode (as opposed to a continuous mode discussed hereinafter) in which pulses are transmitted, received, and processed on a cyclical basis. Most types of processing in this mode use a focused ultrasound signal. Areas outside the focus can be ignored, thereby limited the dynamic range of signals that need to be analyzed. The variable gain amplifier 220 limits the signal 200 to a defined dynamic range, without unacceptably affecting the resultant image.
The output of the variable gain amplifier 220 is filtered by a Nyquist filter 222 (in effect a low pass filter) to remove frequencies that can""t be sampled by the A/D converter 224. The highest frequency which can be accurately represented is one half of the sampling rate. Therefore, the Nyquist filter 222 is selected to match A/D converter 224 which converts the output of the Nyquist filter 222 into the digital domain. Current ultrasound systems employ an 8-bit or 10-bit converter.
After being converted into a digital signal, the output of the transducer array is processed through beamform logic 226. Basically, the beamform logic 226 delays the digital output of each channel by a predetermined amount (based on the desired direction and focusing of the receive beam shape). One method of accomplishing this is to load the output into a register and after the predetermined time reading the register. Subsequently, the output of all of the channels are summed by summing logic 228.
Each transmit event on the transducer array 114 (FIG. 1) starts a receive, process, delay and sum cycle in the receive waveform generator 118. The resultant digital representation of the echo of each transmit event is submitted to digital processing 230, such as signal demodulation and scan conversion.
As noted above, certain types of processing are currently performed in the analog domain. Perhaps the best example is Continuous Wave Doppler processing (CW processing). Doppler processing, in general, seeks to determine the speed of blood flow through vessels. Currently there are two types of Doppler processing, Pulse Wave and Continuous Wave.
Pulse Wave Doppler processing employs the pulse mode scanning described above. Returning echoes are analyzed (using Doppler shift) to determine a speed of blood through the target vessel. This analysis is limited by the Pulse Repetition Frequency (PRF) of the ultrasound system. Because sound does not travel instantaneously, the repetition frequency of a transmitted pulse is limited by the amount of time it takes for each pulse to return. Another pulse typically cannot be transmitted until the echo of the prior pulse is received. The PRF for most ultrasound systems is approximately 8 KHz. As this technique is, in effect, a sampling technique, the Nyquist theorem applies, such that Pulse Wave Doppler techniques can only measure peak velocities for Doppler shifts up to one half of the PRF, in this case 4 KHz. This is generally adequate for normal blood flows, but some conditions lead to blood flows that create a frequency shift up to 20 KHz, requiring a PRF of 40 KHz. For some of these conditions the only medical measurement upon which a diagnosis can be comfortably predicated is the speed of blood flow.
CW processing was primarily created to measure high velocity blood flow. In CW processing, a transducer""s elements are divided into two groups, a send group and a receive group. The send group of elements continuously transmit an ultrasound wave while the receive group continuously receive echoes. The Doppler shift of the received signal is measured and a velocity is determined. Of course, because echoes cannot be matched with transmissions, any idea of range (i.e. depth) is lost and the output is a simple velocity measurement. As depth is indeterminate, the 160 dB entire dynamic range of the return signal must be analyzed to find the greatest Doppler shift. The dynamic range significantly exceeds the dynamic range of 8-bit or 10-bit converters, 121 dB and 133 dB respectively. Accordingly, such analysis is performed in the analog domain.
In the analog path, each signal is first passed through an anti-alising filter 232 prior to being delayed by a phase delay circuit 234. The output of each element in the transducer array 114 is then summed by a summing circuit 236. Subsequently, the summed signal is submitted to analog processing 238, such as CW processing. The result of the analog processing 228 is subsequently A/D converted by an A/D converter 240 in preparation for digital processing 230, including scan conversion and display.
The phase delay circuit 234 generally comprises analog delay lines which are bulky and expensive. Current 1-D transducer arrays have 64 to 512 elements, while the new 2-D arrays for use with 3-D imaging have thousands of elements. Accordingly, the cost and size of the analog circuits will become an obstacle to 3-D processing in the analog domain. Further, it is inconvenient, and time consuming, to incorporate the analog data into a digital display. Accordingly, there exists a need for an ultrasound system that can perform beamforming and signal processing (including CW processing) entirely in the digital domain.
An ultrasound system is described that performs image processing, including CW Doppler, in the digital domain. The ultrasound system is provided with a transducer having a plurality of elements and an ultrasound receiver with a plurality of channels. Each channel receives an analog echo signal and outputs a digital representation of the analog echo signal using an AID converter capable of converting a signal with a dynamic range of at least 160 dB at twice the Nyquist rate. As such an A/D converter can represent the entire useful dynamic range of an ultrasound echo signal, operations, previously performed in the analog domain, can be performed in the digital domain. These operations include filtering the data for harmonic signals and CW Doppler processing.